1. Field of the Invention
The present invention relates to a signal processing apparatus and method of a differential transformer for obtaining a measurement signal having a fast response speed from a differential transformer having a low excitation frequency. More particularly, the present invention relates to a signal processing apparatus and method of a differential transformer suitably employed in a high-resolution electric micrometer having a resolution of approximately 1 nm, in which the response speed of a measurement signal can be increased by a factor of 10 to 100 (200 Hz to 2 kHz) with the use of a conventional head having an excitation frequency of approximately 5 kHz.
2. Description of the Related Art
A conventional electric micrometer has been configured as disclosed in Japanese Patent Laid-Open Publication No. 2002-340505. In the electric micrometer includes, as shown in FIG. 1, the displacement of a spindle 22 which is a part of a head 20 is transferred to a core 26, and sensor coils 28 and 30 are placed symmetrically with respect to the mechanical center point (neutral point) of the core 26 and are connected each other in series to form a differential transformer. When a voltage is applied to the sensor coils 28 and 30 through an external oscillator 32, voltages E1 and E2 are generated at the ends of the sensor coils 28 and 30, respectively, as shown in FIG. 2 due to the impedance change of the sensor coils 28 and 30 according to the position of the core 26. The displacement of the spindle 22 is detected through the voltage difference (E1−E2) based on the change in the position of the core 26.
Specifically, as shown in FIG. 3, a sine-wave driving signal having a frequency of, for example, 5 kHz generated in the oscillator 32 is applied to the sensor coils 28 and 30 (not shown in FIG. 3) of the head (cartridge head or lever head) 20 via a driving amplifier 34. The output signal obtained from the neutral point of the sensor coils 28 and 30 is amplified by a preamplifier 36, and the amplified signal is half-wave or full-wave rectified by means of a synchronous detector 38 in synchronization with the excitation signal generated by the oscillator 32. The 5 kHz component of the rectified signal is removed by a low-pass filter (LPF) 40, and the filtered analog signal is then displayed on a meter. Alternatively, the filtered analog signal is converted into a digital signal by means of an analog-digital (A/D) converter 42, and the converted digital signal is digitally processed by a microprocessor 44 to digitally display. The digital data output signal output from the microprocessor 44 is subjected to processing by means of, for example, a parallel-serial converter, a shift register, a gate, a communication driver, and the like in accordance with need.
However, since the electronic micrometer is composed of the differential transformer (inductor), the balance adjustment for the capacitance component and also for the inductance component cannot be made with the use of a resistor. Therefore, the signal at the reference position cannot be accurately adjusted to zero. In addition, the excitation frequency cannot be increased due to the characteristics of the head 20, for example, it is at most 5 kHz. Therefore, a ripple component having a frequency of 5 kHz and generated at the synchronous detector 38 remains in the subsequent stages of the circuit. Therefore, the LPF 40 having a low cut-off frequency must be provided after the synchronous detector 38. However, this causes a reduction in the response speed (for example, approximately 20 Hz (−3dB) in a low-end product having a resolution of 0.1 μm). Further, in a high-precision electronic micrometer which requires a resolution of approximately 1 nm, the response speed becomes much slower (several hundred milliseconds (several Hz)).
Although the above problems may be solved through increasing the excitation frequency of a head, modifications to the head means that a newly designed head must be developed.